The present invention generally relates to a semiconductor laser unit, and in particular to a semiconductor laser unit which is detachably provided, as a light source for optically writing information in a recording medium, into an optical system such as a laser printer, a copier, and a digital fascimile machine.
A semiconductor laser unit is used as a light source and is detachably provided into an optical system such as a laser printer. Generally, a semiconductor laser unit includes a laser diode for emitting a laser beam which has been modulated, and a collimator lens used for changing the emitted laser beam to a parallel laser beam. The parallel laser beam emitted from the semiconductor laser unit passes through scanning means and image-formation means, and scans a photosensitive member such as a photosensitive drum, so that an image can be formed on the photosensitive member on the basis of the principle of electrostatic recording.
A characteristic defined by a relationship described below is an important factor which decides performance of the optical information recording. The above is a relationship between emission power or light intensity of a laser diode and an output voltage of a monitor output amplifier which converts a current signal derived from a photodiode for monitoring light emitted from the laser diode to a voltage signal. Generally, the photodiode is packaged with the laser diode. The above characteristic changes due to variation in an angle of divergence of the laser beam, location and sensitivity of the monitor-use photodiode, and transmittance of the collimator lens. The presence of the above variation causes change of power of the laser beam obtained on the photosensitive drum even when a fixed bias current is applied to the laser diode. For this reason, it becomes necessary to obtain a predetermined light intensity by adjusting the bias current supplied to the laser diode, while monitoring the light intensity of the laser beam by using a power meter, which is placed in a vicinity of the photosensitive drum. It is noted that the bias current is varied by adjusting a variable volume which is mounted on an external printed circuit board which is provided separately from an internal printed circuit board provided into the semiconductor laser unit.
However, the above adjustment must be carried out every time when the semiconductor laser unit being used is replaced with a new one. Therefore, the adjustment is very difficult. Additionally, the adjustment is complex, because the power meter must be actually placed in the vicinity of the photosensitive drum provided in the laser printer. Further, a long time is taken for the adjustment operation. During the operation, the used of the laser printer is prevented.
An improved semiconductor laser unit has been proposed in the Japanese Laid-Open Patent Application No. 61-174794. The proposed semiconductor laser unit includes a monitor output generator consisting of an amplifier and a variable resistor for adjusting gain of the amplifier. The monitor output generator is mounted on the internal printed circuit board included in the semiconductor laser unit. An output signal of the amplifier is supplied, through a cable, to a comparator which is mounted on the external printed circuit board. A reference voltage signal which corresponds to the predetermined light intensity and which is produced by a reference voltage generator mounted on the external printed circuit board, is supplied to the comparator. The bias current to be supplied to the laser diode is adjusted so that the output of the comparator becomes equal to zero, and thereby the light intensity of the laser diode is set to the predetermined light intensity which corresponds to the reference voltage. The adjustment is carried out during a manufacturing process of the laser printer.
However, the proposed semiconductor laser unit has disadvantages described below. Actually, it is difficult to precisely set the reference voltage signal to a predetermined voltage level. In other words, the reference voltage for each laser printer is not identical to each other. Further, it is very difficult to keep the reference voltage signal within a tolerance during long-term use. For example the reference voltage must be kept within a tolerance of .+-.1% with respect to a reference voltage of 2.5 volts. As a result, when the semiconductor laser diode is replaced with a new one, the gain adjustment of the amplifier is necessary to obtain the predetermined light intensity on the photosensitive drum by using the power meter. In practice, a reference voltage which is generated by an adjustment instrument is supplied to the comparator, in order to facilitate the adjustment operation. However, generally, the reference voltage generated by the adjustment instrument is not equal to the reference voltage generated by the reference voltage generator mounted on the printed circuit board in the laser printer. For this reason, it is actually difficult to adjust the driving current so as to obtain the predetermined light intensity. Moreover, the output signal of the monitor output amplifier is sent to the external circuit provided on the external printed circuit board through a cable connected therebetween. Since the output signal of the monitor output amplifier is an analog signal, it is liable to be affected by the presence of noise.